Viscosity measurement of liquids is an essential tool used in the characterization of many products. For instance, the suitability of lubricating oils for a particular purpose is closely related to viscosity and to the variation of viscosity with temperature. Most polymers, including nylon, polyester, rubber, polyethylene, cellulose, polyvinylchloride, and polystyrene are routinely characterized during production by dissolving them in a suitable solvent and then measuring the viscosity of the resulting solution.
The viscosity of liquids is strongly dependent on temperature. As the temperature of a liquid changes, the viscosity also changes. An accurate measurement of viscosity therefore requires accurate and stable temperature control. If viscosity is to be measured to within an accuracy of 1%, the temperature must be highly controlled.
Measurement of viscosity is often performed in thermostatic baths using a liquid as the thermostatic media. Such baths often have an accuracy of +/-0.01.degree. Celsius. This method relies on the high heat capacity of a liquid to provide a stable temperature environment during viscosity measurement. In the typical viscosity measurement apparatus, a vessel of water or oil is stirred and heated using a highly accurate thermostatic controller. In such a vessel the rate of change of the temperature is low and the viscosity measurement can be performed with relative ease. Liquid baths, however, have several disadvantages. For instance, a liquid bath requires constant maintenance. If water is used as the bath medium, evaporation can pose a problem. Bacterial and algae growth in water necessitates frequent changes of the water media.
When higher temperatures are used, oil is normally chosen as the liquid medium. As the oil becomes hot and starts to oxidize, an objectionable odor is created in the laboratory. Another disadvantage of oil baths is the risk of dripping the oil media onto laboratory surfaces once the viscosity measurement apparatus is removed from the bath. Also, liquid baths are subject to leaks. In the case of hot oil baths, operators are exposed to the danger of burns in the event that the bath container, which often is glass, is broken.
An additional disadvantage with using a liquid medium as a thermostatic bath arises when viscosity must be measured at several temperatures. A separate thermostatic bath must then be maintained at each temperature. Due to the high heat capacity of liquid, the time required for changing temperature in a bath is between six to eight hours. Typically, a lubricant laboratory would have three to four liquid baths operating continuously.
It thus is seen that the liquid baths utilized in the prior art as part of viscosity measurement apparatuses require high maintenance and care to operate. Additionally, numerous baths are required to measure a wide range of temperatures.
Accordingly, a need remains for a viscosity measuring apparatus and method which can accommodate a wide range of temperature measurements and yet which is not susceptible to the problems heretofore associated with liquid bath type apparatuses.